This invention relates to improvements on a voltage-controlled oscillator.
In a prior art voltage-controlled oscillator, a capacitor is charged by a constant current Ic which is controlled by a control voltage Vc. Reaching of the charged voltage to a predetermined voltage V.sub.th is detected by a comparator and thereupon a monostable multivibrator is actuated to bring a transistor parallelly connected to the capacitor into conduction and thereby to discharge the capacitor. According to this prior art voltage-controlled oscillator, electric charge up to which the capacitor is charged is constant regardless of the control voltage Vc, and therefore the control voltage Vc controls the charging time T only. If capacity of the capacitor is represented by C and reference voltage by V.sub.ref, the electric charge to be charged in the capacitor is C(V.sub.ref -V.sub.th), and the charging time is ##EQU1## (where R represents resistance and V.sub.c /R=1c). On the other hand, discharging time .tau. is determined by the pulse width of the monostable multivibrator. Thus, the oscillation frequency f is represented by an equation f=1/(T+.tau.). The discharging time .tau. is constant regardless of the oscillation frequency f and, accordingly, the discharging time .tau. cannot be disregarded if the oscillation frequency f is high, though .tau. can be neglected if the oscillation frequency f is low. Consequently, there arises an inconvenience that the oscillation frequency f is not exactly proportional to the control voltage.